In nuclear spectroscopy applications, the energy of incident nuclear particles is measured. In many cases this measurement is accomplished by measuring the energy deposited by the particle in a nuclear detector. The energy is from a continuous pulse train that is in theory infinite in length. To obtain a signal that represents the total energy deposited in the detector typically requires the integration of a current signal. In general, a compromise between the charge integration time and the need to process a high pulse rate has to be found. Generally, the detection of a series of distinct pulses, at low count rate, offers very few problems, therefore, conventional detection equipment may be employed. However, sometimes the frequency or repetition rate of the pulses varies over a wide range such that the spacing between successive pulses is sometimes very short. Thus, the random character and high rates of occurrence of these signals necessarily produce a "pile-up" or a sequence of overlapping pulses at the amplifier unit. Usually, a pile-up of this sort results in a single pulse that is composed of two or more amplified individual detected signals, each of which is indicative of a detected gamma ray, neutron or other nuclear radiation. The pile-up phenomenon results in data losses and/or spectrum distortion. Accordingly, it is of importance, first to distinguish individual pulses from pile-up pulses and second when pile-up pulses have been detected, to adequately process these pulses in order to restore the original distinct pulses, or at least to reject pulses which are the result of pile up and do not represent the energy deposited by a single particle in a nuclear detector.
These pulses are analyzed using a nuclear spectrum analyzer or pulse height analyzer. A nuclear spectrum analyzer may include a scintillation detector, a photomultiplier, a coupler (usually a capacitor), a preamplifier, a pulse shaping unit and a pulse height analyzer. Known pulse height analyzers comprise successively a pulse detector (optionally a pile-up detector and a pile-up process unit), an analog-to-digital converter (ADC) and a memory, the different channels of which correspond to a given amplitude level of the detected pulse. The pulse height analyzer may also comprise, upstream of the ADC, an input gate preventing pulses from reaching the ADC when the latter is busy, i.e. when the ADC is processing a detected pulse.
As previously stated, if two pulses arrive within the same integration interval, a biased "sum" pulse (pile-up) will be generated. This pile-up results not only in spectral distortion but in the failure to detect one of the pulses. Pulse shape can also affect the detection of pile-ups. Therefore, a compromise between integration time and count rate capability is made by a pulse shaping device which precedes the digitization of the pulse and its accumulation in the spectrum (memory). New high speed electronics provide high throughput nuclear spectroscopy acquisition. However, such systems are more sensitive to changes in the pulse shape and require that the shape of the electronic signal remain unchanged over a large range of operating conditions. The main factor which can influence the shape of the pulse is the change of the characteristics of electronic components and nuclear sensors over time and/or with temperature.
One approach to spectral analysis is a digital integration technique described in U.S. Pat. No. 5,067,090, issued to Bronislaw Seeman. In this nuclear spectroscopy technique, pulse height analysis is performed for a pulse with an amplitude that is a measure of the energy of particles, such as gamma rays, collected by said radiation detector by (1) continuously (asynchronously) converting the detected signal to digital samples at a given rate; and (2) processing each of the digital samples so as to form a digital image of each detected nuclear event. This method includes the step of detecting the arrival of a pulse, by comparing each incoming sample to a threshold value, so as to determine whether the sample is representative of a pulse. More specifically, the difference between the incoming sample value and the base signal which is free of nuclear events, is calculated and compared to the threshold. The time of arrival of any detected pulse is recorded. Moreover, the base signal value (baseline) is continuously estimated and updated at each sample time arrival, so as to generate a current base signal value; preferably, the updated value is a weighted average of the incoming sample with the preceding sample or samples. Once a pulse has been detected, the energy of said pulse is calculated by summing the difference between each sample value (representing the pulse) and the current base signal value, the sum being continuously accumulated in a register. During the energy calculation, the accumulated sum is compared to a preset value and said register is reset in case of overflow. Advantageously, for each pulse, the sample just proceeding the first sample representative of the pulse, as well as the sample just following the last sample representative of the pulse, are both taken in to account for said pulse energy calculation.
Seeman further includes the step of detecting a stack of successive pulses close one to the other in time. The detection step involves counting the number of samples representative of a detected nuclear event and comparing the count to a predetermined maximum count value. The sampling step can be performed by a flash analog-to-digital converter (ADC).
The digital integration technique in Seeman, which in essence computes the area under the pulse is sensitive to the shape of the pulse. The pulse shape is influenced by changes in electronic components with temperature or age as well as by changes in the pulse response of the nuclear detector. Most scintillation detectors will emit a much more rapid pulse at elevated temperature than at room temperature.
A possible technique for stabilizing the pulse shape is described in U. S. Pat. No. 5,132,540 issued to R. Adolph and B. A. Roscoe. This method regulates the pulse shape by keeping track of the number of pile-up pulses in a given relationship and comparing that number to the total number of counts. This method has the disadvantage of being slow and unreliable at low count rates, where the total number of pile-up pulses is very small.
Other applications in nuclear measurements require discrimination between interactions by distinguishing the shape of the pulse from the nuclear sensor. At the present time, most pulse shape discrimination relies on complex and often unstable analog circuits. Unfortunately, the use of these techniques requires substantial expertise. A need still remains for a simple method that can distinguish between pulses of various lengths and pile-up pulses.